This invention relates to the preparation of fiber suspensions for use in the making of paper, and while it was developed in connection with the preparation of waste paper products for recycling, it is also applicable to the treatment of wood pulp chips and other cellulosic fibrous materials.
The recycling of waste paper products in significant volume is much older than any patent now in force, but the most significant increase in its practice followed World War II when a major increase in the use of corrugated board containers made comparatively large amounts of used cartons available for recycling. Prior to that development, the waste paper products primarily subject to recycling were newspapers and other printed paper, which required deinking as a major stage in their preparation for reuse.
Large quantities of used corrugated cartons which became available for recycling following World War II posed a different preparation problem in that a great many had been assembled with the aid of asphaltic adhesives that were impossible to remove by any commercially feasible process. Further, the asphaltic adhesives tended to appear as substantial globs or smears on the surface of paper products made therefrom, and their use was therefore limited to low grade products such as low grade boxboard.
These problems were relatively successfully avoided by a technique which became known as "asphalt dispersion" by which the asphaltic materials were reduced to such small particle size and were so thoroughly dispersed in the stock that it could be used successfully in a wider variety of boxboard-type products, especially when provided with a cover layer of better grade stock. United States patents disclosing asphalt dispersion technology include Hollis U.S. Pat. Nos. 2,697,661 of 1954, and 2,977,274 of 1961, and Sandberg U.S. Pat. No. 3,057,769 of 1962.
Each of those patents taught that a preliminary step according to its technique was pulping the waste paper products in a conventional manner, followed by digestion under steam pressure to soften the thermoplastic asphaltic materials, and then by subjecting the digested pulp to a refining action, and all three patents show this action being carried out in a disk-type refiner. Each of these patents also taught that the system should include a cyclone for separating the steam from the digested pulp.
A somewhat different approach was proposed in another patent contemporaneous with Sandberg and the second Hollis patent, namely Durant U.S. Pat. No. 2,910,298 of 1959. In the system and method disclosed by Durant, the initial pulping operation was replaced by steps of shredding the waste paper material, cleaning the shredded material to the extent of removing tramp metal and other foreign objects by air-float cleaner apparatus, and then moistening the cleaned waste paper pieces before feeding them into a steam pressure digester. At the discharge end of the digester, Durant provided a different form of refining apparatus where in effect the digested waste paper was subjected to a combing action under steam pressure. The ultimate object was the same as in the Hollis and Sandberg patents, namely to reduce asphaltic contaminants into particles of minimal size and to distribute those particles throughout the fibers.
More recently, a joint development by Wisconsin Tissue Mills and Stake Technology Ltd. was described in a paper entitled "Steam Explosion Technology and Fiber Recycling" presented at a TAPPI conference in March of 1991. The system described in that paper and related literature includes a digesting chamber where high temperature and pressure are maintained throughout the dwell time, following which the cooked waste paper materials are discharged into air at atmospheric pressure and room temperature. The paper lists three runs in which the temperature range was 190.degree. to 203.degree. C., the dwell time was four minutes, and the pressure was approximately 400 psi. The process is described as resulting in particle sizes of residual contaminants of from 1/2 to 1/10 that of the contaminants in waste paper furnish produced by repulping without steam treatment.